The invention pertains to a circuit to test the working of an antenna, especially an antenna for a radio telephone that has more than one antenna. The invention allows the radio telephone to detect a malfunction in the antenna wire or detect a missing, incorrectly mounted or failed vehicle antenna, for example as the result of damage from an accident, at any time and to automatically switch to a operative antenna.
Due to the fact that a radio telephone only works when all components of the communication system work and that antennas are often mechanically sensitive due to their location, the solution according to the invention significantly increases the reliability of a radio telephone in an emergency.
Telephones in vehicles are usually equipped with an external or window mounted antenna. The location of this antenna is primarily determined by the requirements that need to be met to achieve optimal transmitting quality.
One disadvantage of selecting such a location is that the probability of damaging the antenna to the point of total failure is high the vehicle is involved in an accident or when other external forces act on the antenna. In particular, these other external forces acting on external antennas include, for example, the intentional destruction of the antenna by a stranger or the breaking off of the antenna while passing under an obstacle with low clearance. The total failure of the antenna can have fatal consequences in a traffic accident or when the vehicle is damaged as it is not possible then to make a telephone call in order to call for help.
To eliminate this imperfection an emergency or back-up antenna is installed in a different location as stated in publication EP 0 859 237-A1. This secondary antenna is then used for sending/receiving after the external antenna used as the main antenna fails. Each antenna is connected to the radio telephone via a separate coaxial cable.
To obtain the maximum transmission quality and to prevent interference during communication, the emergency antenna is not in operation while the main antenna is working. This means that the emergency antenna and the corresponding wire are only to be put into operation in an emergency by the manual or automatic initiation of an emergency call. To accomplish this, an emergency call button is activated or the air bag and/or seat belt mechanism controller sends a corresponding control signal to the radio telephone when switch over the radio telephone to the secondary antenna connection.
In principle, there are various solutions used to switch the radio telephone to the emergency antenna:
In simple solutions, the initiation of an emergency call in the radio telephone will automatically force the radio telephone to switch to the connection for the emergency antenna regardless of whether or not the main antenna is still operational. One requirement for this to occur is that there must be a high probability that the emergency antenna and its separate antenna wire still working due to installation in a protected location.
However, malfunctions or damage to the antenna feed cable leading to the emergency antenna can arise when operating the vehicle or connecting the antenna during the manufacture of the vehicle that remain undetected because the emergency antenna is not used during normal operation. Under certain circumstances, this antenna may not work properly in an emergency. Additionally, its efficiency is generally lower that that of the main antenna when installed in the interior of the vehicle. This may also lead to the inability to connect to the base station using the less powerful emergency antenna when the vehicle is in an unfavorable position although the connection could be made using an intact main antenna.
To avoid this disadvantage, radio telephones with several antenna connections and other accessories periodically perform a test procedure in which the antennas are operated alternately and tested to see if they are working properly. This can be done, for example, by comparing the signal strength of the signal received or, in accordance with publication EP 0 859 237 A1, by comparing the signal strengths of the signal supplied and the signal reflected back by the antenna. In this manner, malfunctions and damage to the antennas and the wires will be detected and indicated, and the unit can quickly switch to a working path of the antenna. The test procedure is also generally performed when an emergency call is triggered so that the unit only switches to the less powerful emergency antenna when the main antenna has failed due to the whip being broken off, for example. When both antennas also have different reception results due to having different designs and locations, this method is not very reliable due to the unequal intensities of the signals received.
For a test procedure according to the publication EP 0 859 237 A1 the antenna matching is measured by determining the reflection factor on the antenna wire with a bi-directional measuring coupler and a circuit to produce the quality signal. A disadvantage of this solution is the complexity of the hardware and software used to implement the test procedure.
In addition, there is already a device for testing vehicle antennas in publication DE 196 27349-A1 that constantly monitors vehicle antenna receiving coils in current loops with a low idle test current. In a rail car the receiving coils receive inductive signal currents as input along a conductor such as the tracks or the overhead wires. The idle test current is preferably a DC current and continuously shows that all antennas on the vehicle are present as well as connected.
One disadvantage of this, however, is that these vehicle antennas are not the type of antenna preferred for use in motor vehicles, such as an rod aerial fed asymmetric, but are in the form of receiving coils used to inductively detect signals. Therefore the solution can only be used for motor vehicles when the well-known folded dipole antenna with a loop radiator element is used instead of the previously used rod or dipole antenna with a pole radiator with their inherent advantages. This is more complex in comparison to the solutions used and does not provide any significant advantages for the intended application. Another disadvantage of the known solution is that a short-circuited antenna wire will also be displayed as a working antenna.
It is therefore the task of invention to create a simple and economical circuit to test the working of at least one antenna for a radio telephone that avoids the shortcomings stated and can be used regardless of the shape or type of the antenna for the most part. In addition the invention should uniquely identify different types of possible connection errors when connecting several antennas to a radio telephone.
The solution according to the invention contains an antenna with an open radiator such as a pole radiator, for example. The antenna has one end on which an antenna wire is connected for detecting or supplying the RF signal and a second end that projects into space so that the capacitance of the rod distributed in space creates an RF path that closes the signal circuit for communication purposes. To accomplish the task the radio telephone sends a test current to the antenna via the antenna wire. This is independent from the signal current. The test current is preferably a DC current or an AC current with a wavelength that is many times longer than the wavelength of the signal current.
According to the invention there is a secondary path with an impedance connected to the radiator that creates a return path for the test current flowing to the antenna wire and that is parallel to the RF path. The test current causes a drop in voltage across this impedance.
In contrast to the known solution the circuit contains a voltage evaluator that constantly monitors the voltage on the antenna connections of the radio telephone that arises due to the test current flowing through the impedance. In this manner the radio telephone not only detects if the pole radiator is correctly connected to the antenna connection, but also if there are any short circuits in the antenna wire.
The impedance value of the secondary path is many times higher than the radiation resistance of the antenna for the signal current as well as for the test current. The impedance is connected to the radiator by a connecting wire that is short in comparison to the transmission wavelength.
According to a special feature of the invention, the impedance of the secondary path consists of an elongated structure whose length, when it is a single component, for example, is of the same order of magnitude as the length of the rod antenna, or it consists of several discrete elements connected in series so that the connection wires to each element in the secondary path are short in comparison to the operating wavelength and have as little effect as possible on the RF characteristics of the radiator.